The invention relates to a method for continuously filtering raw brine for use in chlor-alkali electrolysis by means of a backwashing pressure filter.
During conversion to membrane cell technology in chlor-alkali electrolysis the demands on the brine qualities have increased considerably. In order to protect downstream plant components and electrolysis cells, inter alia solid, suspended impurities have to be removed down to a very low level. This solid/liquid separating task is frequently carried out nowadays by multi-stage and correspondingly expensive separating methods, high demands being placed on the reliability primarily in the last separating stage.
Owing to the high investment and operating costs for such plant, alternative concepts which allow method simplification are becoming increasingly important. In addition, the fact plays a part that many installations are conversion operations replacing available obsolete electrolyses by the more efficient and environmentally friendly membrane plant. In these cases an extensive use of available separating apparatus is aimed for. The local conditions, such as infrastructure, space limitations; etc. also have to be considered here. The shutdown or disassembly of available separating apparatus (primarily large-volume thickeners) is often also not desirable at all or is-connected with very high expenditure.
Raw brine contains insoluble suspended solids originating from dissolving basins or precipitation reactions. Typical components of raw brine are barium sulphate, calcium sulphate, calcium carbonate, metal hydroxides and gangue. The concentration and composition of the impurities depends primarily on the salt source and can vary sharply depending on location. Typical concentrations for brines made of rock salt of European origin are in the region of 300 to 1,500 ppm suspended solids.
Owing to the decanting primarily fine particles or small agglomerates arrive in the overflow while primarily larger particles or agglomerates are to be found in the underflow. For this reason the direct filtration of the overflow is difficult on backwashing pressure filters or is sometimes even impossible. The fine particles quickly tend to form a thin and impermeable layer on the filter medium. An immediate increase in pressure and low filtration rates result. In addition it is difficult to remove the particle layer and this leads to a sharply limited backwashing capacity (capacity to regenerate) of the filter medium.
A method for purifying a backwashing pressure filter is known from U.S. Pat. No. 4,443,346 of the type which is suitable, for example, for use in the method according to the invention.
U.S. Pat. No. 3,497,452 describes a method using auxiliary filtering means, some of the filtrate being reprocessed for further filtration, in the removal of solids from solvents in the chemical industry.
Although the formation and removal of a filter cake is described in known methods, it is not described how or in what form the auxiliary filtering means is used. As is known, substances extraneous to the process of mineral (for example perlite, diatomite) or organic (for example cellulose) origin are added as auxiliary filtering means in suspended form to the prefilt prior to the filter. The drawback is that in known applications new auxiliary filtering means have to be added and accumulate as an additional residue during disposal.